In British Pat. No. 1,409,768 there are described several heterocyclic derivatives of guanidine in which the heterocyclic moiety is a 5- or 6-membered saturated 1,3-diazacarbocyclic-2-ylidene. These derivatives are unsubstituted on the imino nitrogen of the guanidine moiety. In contrast, the compounds of the present invention differ by being a heterocyclic derivative of guanidine which carries a bulky substituent on the imino nitrogen of the guanidine moiety. Additional prior art, but further related, may be represented by German Offen. Nos. 2,321,330 and 2,502,397; U.S. Pat. Nos. 3,914,306, 3,933,836 and 4,073,636; and British Pat. No. 1,341,245.